1. Field of the Invention
The invention relates to a hydraulic control apparatus that controls engagement of a clutch provided in parallel with a fluid transmission apparatus on a path for transmitting power output from a power source.
2. Description of the Related Art
In a conventional transmission, the transmission is disposed on a power transmission path extending from an engine to a vehicle wheel in order to transmit the power of the engine to the vehicle wheel, and an automatic transmission is available in which a multi-stage transmission including a planetary gear mechanism is used as such a transmission, and a gear ratio of the transmission is switched by operating a frictional engagement apparatus such as a clutch or a brake. A continuously variable transmission employing a belt and a pulley is also available as a transmission. A technique of controlling the gear ratio of this type of automatic transmission using a hydraulic actuator is available, and an example of this technique is described in Japanese Patent Application Publication No. 2006-307950 (JP-A-2006-307950). In JP-A-2006-307950, an automatic transmission is connected to an output shaft of an engine via a torque converter. Further, an oil pump driven by the engine and an oil pump driven by an electric motor are provided, and the gear ratio of the automatic transmission is controlled by supplying pressurized oil ejected from the oil pumps to a hydraulic actuator. Furthermore, of the pressurized oil ejected from the oil pumps, surplus oil supplied to the hydraulic actuator is supplied to a lubricating system of the automatic transmission through a downstream oil passage. A warmer is disposed in the downstream oil passage such that at low temperatures, the pressurized oil is warmed by the warmer. As a result, supply delays in the working oil supplied to the hydraulic actuator may be reduced, and a favorable running performance can be obtained. Note that techniques for warming the working oil of a transmission are also described in Japanese Patent Application Publication No. 10-159927 (JP-A-10-159927) and Japanese Patent Application Publication No. 4-75424 (JP-A-4-75424).
Meanwhile, a vehicle in which a fluid transmission apparatus and a lockup clutch are provided on a power transmission path extending from a power source of the vehicle to a vehicle wheel is also available. When the lockup clutch is not engaged, slippage occurs in the fluid transmission apparatus, leading to power loss. Therefore, power loss may be avoided by engaging the lockup clutch. However, if the operating responsiveness of a switching valve for engaging and disengaging the lockup clutch is relatively low, vibration may occur during engagement of the lockup clutch even if working oil supplied to a hydraulic chamber for controlling the engagement of the lockup clutch is warmed.